Voltage-controlled oscillators (VCO's) and digitally-controlled oscillators (DCO's) are used to generate a signal having an oscillation frequency determined by a control signal. In a VCO, a fine tuning component of the control signal is specified using an analog control voltage, while in a DCO, the fine tuning component of the control signal is specified using a digital control signal. To save power in electronics devices such as portable communications devices, VCO's and DCO's are increasingly designed to work with power supplies having lower voltage levels.
In one prior art oscillator circuit design, an LC tank having a variable capacitance is coupled to at least one cross-coupled transistor pair. The cross-coupled transistor pair functions as a negative resistance, causing the voltage across the LC tank to oscillate at the tank resonant frequency. In prior art oscillator designs, the transistor drains may be directly DC cross-coupled to the transistor gates. This DC cross-coupling reduces the voltage headroom available from a low voltage power supply, since the drain-source voltage is made equal to the gate-source turn-on voltage of the transistors. In a CMOS cross-coupled pair oscillator design, wherein both an NMOS and a PMOS cross-coupled pair are provided, the voltage supply must support both the NMOS gate-source turn-on voltage and the PMOS gate-source turn-on voltage.
It would be desirable to provide techniques for oscillator design that more efficiently utilize the voltage headroom available from a low voltage power supply, while adequately meeting other oscillator design criteria such as low phase noise.